The present invention relates to a method and apparatus for manipulating line weight in an image representation, and more particularly, a method and system for spreading the contour lines in slope mapping, wherein a camera lens is orbited in a plane parallel to the image.
A slope map, defined as one which depicts bands or regions of terrain wherein the terrain slope is greater than or equal to a defined slope, is particularly useful in studies related to land use. Typically, up to five or six such bands in different colors are displayed on the slope map representing, for example, terrain slopes ranging between zero and 5%, 5% and 15%, 15% and 30%, 30% and 50%, 50% and 70%, and slopes greater than 70%. The map is prepared at a predetermined scale, e.g., 1:24,000.
Generally, a slope map is prepared from a contour map negative containing contour lines which represent constant altitude regions, with adjacent contour lines being at a predetermined altitude differential, e.g., ten feet (the contour lines are closer together in steep terrain, than they are in relatively flat terrain). In order to form slope bands, the line thickness on the contour map negative is increased by an amount which depends upon map scale, contour interval and desired slope band. As the contour lines are thickened, contour lines spaced relatively close to each other coalesce to form a slope band representative of a region having a slope of at least a preselected value, depending on resultant line thickness. For example, where a relatively high slope band (representing only steep terrain) is required, e.g., 70% and greater, the contour lines are thickened by only a relatively small amount, and only contour lines that are very close to each other coalesce. On the other hand, where a relatively low slope band (representing relatively flat as well as steep terrain) is required, e.g., 15% and greater, the contour lines are thickened by a substantial amount. In the latter case, even contour lines that are spaced apart by a relatively large distance tend to coalesce.
The process of making a slope map requires first that a second contour map negative be prepared from the original negative with the contour lines thickened by a calculated amount to define the required slope band. Then, with the contour lines thickened, and slope band defined, a positive is made from the second negative. During production of the positive, contour lines outside of the slope band undergo a process known as "choke-back", i.e., are reduced in thickness until eliminated. Finally, another negative is made from the positive, depicting only the desired slope band. The entire process is repeated for each slope band required. In optional additional steps, the original contour lines can be superimposed on the negative, along with planographic details such as roads, houses, and the like.
Perhaps the most critical step in the process of slope mapping described above is the step of spreading (thickening) contour lines to form desired slope band. In order to prepare slope maps depicting low percentage slope regions, e.g., in the range of 5 to 10%, the contour lines must be thickened by a substantial amount (up to one-third inch). Spreading or thickening of the contour lines must be uniform and undistorted in order to present an accurate representation of the slope band with a minimum of touching-up required.
Although generally somewhat satisfactory for limited line weight manipulation, prior art apparatus of which I am aware are unsuitable for wide line spreading. For example, in U.S. Pat. No. 3,508,826 to Grabau, the image of an original copy is projected to a fixed light sensitive film in a camera through a lens oriented obliquely to the image copy. The lens is rotated in order to set up a "wobble" to create line spreading on the film. However, since the axis of the lens is oblique to the image, image distortion is created, particularly at fringe areas in a large field of view. Furthermore, since the lens itself rotates, distortion of the image is created by any eccentricity existing in the lens. These undesirable effects are especially prevalent when line spreading is large because the axis of the lens must be made even more oblique to the image copy.
In U.S. Pat. No. 3,204,544 to Shannon, line spreading is effected by employing a controlled, oscillating, flat tabletop, wherein the table defines an orbit having a preselected, fixed diameter. Shannon's system is useful for relatively little line-spreading; for large line-spreading, however, the system creates anomalies or "frills", that is, spurious edges in the thickened contour lines. The reason for the anomalies and spurious edges in that the table oscillates with a fixed diameter. "Painting" of the thickened line image onto the photosensitive film is incomplete, gaps are left in some of the slope bands, and there is loss of detail at the slope edges. Furthermore, Shannon's system is a contact printing system, and since there is no lens magnification available, the scale of the resultant slope map cannot be redefined or corrected for shrinkage.